Technical Field
The present invention relates to light emitting devices, and more particularly to a light emitting device which includes a light emitting element that emits a blue light, and quantum dots that emit a green light by absorbing part of the blue light emitted from the light emitting element.
Description of the Related Art
Conventionally, light emitting devices that emit white light are known. This type of light emitting device includes a light emitting element that emits a blue light, a green phosphor that emits a green light (or a yellow-green phosphor that emits a yellow-green light) by absorbing part of the blue light emitted from the light emitting element, and a red phosphor that emits a red light by absorbing part of the blue light emitted from the light emitting element. Such a light emitting device that emits a white light is used for various applications, including illumination devices and backlights for various displays, such as a liquid crystal display.
In recent years, light emitting devices having all or part of the phosphor replaced by quantum dots (QDs) have been developed. A quantum dot is a semiconductor particle having a diameter of several nanometers to tens of nanometers, and can absorb light such as a blue light emitted from a light emitting element and emit a light different from the absorbed light, as seen in a phosphor. A light emitting device which includes green quantum dots to absorb a blue light emitted from a light emitting element to emit a green light and red quantum dots to absorb a blue light emitted from the light emitting element to emit a red light is also known. For example, JP 2008-544553 A discloses a light emitting device that includes a yellow-green phosphor and red quantum dots.
The quantum dot features a sharp emission peak, that is, a small (narrow) full width at half maximum of the emission peak. Thus, the light emitting device using the quantum dots has an advantage of a wide color reproducibility range when combined with a color filter of a liquid crystal display or the like. Further, matching the peak wavelength of the color filter (a wavelength at which its transmittance reaches a peak) to the emission peak of the quantum dots allow for more light to pass through the color filter, which improves the light extraction efficiency with less attenuation of the light in use of the color filter. In particular, conventional green phosphors and yellow-green phosphors have the respective broad emission peaks. Thus, by use of the green quantum dots, these effects can be remarkably exhibited.
However, these conventional light emitting devices employing the quantum dots are designed to use red quantum dots and may lead to the occurrence of secondary absorption. That is, the red quantum dots may absorb part of green, or yellow-green, light emitted from green quantum dots or a green, or yellow-green, phosphor that has absorbed the blue light, and then emit a red light. The occurrence of such secondary absorption leads to a reduction in the luminous efficiency of the whole light emitting device. Further, in many applications such as displays and illumination devices, there has arisen a need for a light emitting device that can emit brighter light with lower power consumption, that is, which has high luminance efficiency.